A multiple-hearth furnace comprises a furnace wall delimiting a cylindrical space with a vertical axis. A plurality of soles positioned one above the other delimit the hearths of the furnace within this space. In each hearth, rabble arms rotated by means of a central shaft coaxial with the vertical axis of the furnace are provided. These rabble arms are equipped with sole scrapers which turn over the material under treatment on the sole and displace it on a first type of sole toward the periphery and on a second type of sole toward the center of the sole. The first type of sole is provided with peripheral drop holes through which the material under treatment falls onto a sole of the second type in the stage below. The second type of sole is provided with a central drop hole through which the material under treatment falls onto a sole of the first type in the stage below.
It is also a known practice to equip at least one rabble arm in each stage of the furnace with a wall scraper. The function of this wall scraper is to recover the material that accumulates in the immediate vicinity of the furnace wall so as to push it into the peripheral drop holes on the first type of sole and, on the second type of sole, to redirect it into the flow of material being displaced toward the center of the furnace. When the furnace starts, there is a radial clearance between the wall scraper and the inner surface of the furnace wall. However, as the furnace operates, this functional clearance is quickly clogged with material under treatment. A layer of material forms on the inner surface of the wall which the wall scraper progressively compacts by a “pasting” process, eventually forming a very hard crust that adheres to the inner surface of the wall. The wall scraper rubs against this peripheral crust, generating a by no means insignificant additional braking moment on the rabble arm. It should be noted that the situation is aggravated by the fact that hardness and resistance of the peripheral crust are not usually uniform. The modulus of the braking force exerted on the wall scraper thus varies irregularly, causing jerking of the rabble arm. This results in dynamic stresses which generate fatigue effects that are the source of numerous rabble arm fractures.
The object of the present invention is to propose a multiple-hearth furnace which reduces the abovementioned effects. According to the invention, this objective is achieved by a multiple-hearth furnace according to Claim 1.